It is very expensive to drill bore holes in the earth such as those made in connection with oil and gas wells. Oil and gas bearing formations are typically located thousands of feet below the surface of the earth. Accordingly, thousands of feet of rock must be drilled through in order to reach the producing formations. Additionally, many wells are drilled directionally, wherein the target formations may be spaced laterally thousands of feet from the well's surface location. Thus, in directional drilling, not only must the depth but also the lateral distance of rock must be penetrated.
The cost of drilling a well is primarily time dependent. Accordingly, the faster the desired penetration location, both in terms of depth and lateral location, is achieved, the lower the cost in completing the well.
While many operations are required to drill and complete a well, perhaps the most important is the actual drilling of the bore hole. In order to achieve the optimum time of completion of a well, it is necessary to drill at the optimum rate of penetration and to drill in the minimum practical distance to the target location. Rate of penetration depends on many factors, but a primary factor is weight on bit.
Directional drilling is typically performed using a bent housing mud motor drilling tool (known in the art as a “steerable motor”) that is connected to the surface by a drill string. A steerable motor can control the trajectory of a bore hole by drilling in one of two modes. The first mode is called rotary drilling. In the rotary drilling mode, to maintain the trajectory of the bore hole at the existant azimuth and inclination, the drill string is rotated, such that the steerable motor rotates with the drill string.
The other mode is used to adjust the trajectory and is called “sliding drilling.” During sliding drilling, the drill string is not rotated; rather, the drilling fluid circulated through the drill string causes the bit connected to the mud motor drilling tool to rotate. The direction of drilling (or the change in the trajectory) is determined by the tool face angle of the drilling bit. Tool face angle information is measured downhole by a steering tool or similar directional measuring instrument. Tool face angle information is typically conveyed from the steering tool to the surface using relatively low bandwidth drilling mud pressure modulation (“mud pulse”) signaling. The driller (drilling rig operator) attempts to maintain the proper tool face angle by applying torque or drill string angle corrections to the drill string from the earth's surface using a rotary table or top drive on the drilling rig.
Several problems in directional drilling are caused by the fact that a substantial length of the drill string is in frictional contact with and supported by the bore hole. Since the drill string is not rotating in sliding drilling mode, it is difficult to overcome the friction. The difficulty in overcoming the friction makes it difficult for the driller to apply sufficient weight to the bit to achieve an optimal rate of penetration. The drill string also typically exhibits stick/slip friction such that when a sufficient amount of weight is applied to overcome the friction, the drill the weight on bit tends to overshoot the optimum magnitude, and in some cases the applied weight to the bit may be such that the torque capacity of the drilling motor is exceeded. Exceeding the torque capacity of the drilling motor may cause the motor to stall. Motor stalling is undesirable because the motor cannot drill when stalled. Moreover, stalling lessens the life of the drilling motor.
Additionally, the reactive torque that would be transmitted from the bit to the surface through drill string, if the hole were straight, is absorbed by the friction between the drill string and the borehole. Thus, during drilling, there is substantially no reactive torque at the surface. Moreover, when the driller applies drill string angle corrections at the surface in an attempt to correct the tool face angle, a substantial amount of the angular change is absorbed by friction without changing the tool face angle in stick/slip fashion. When enough angular correction is applied to overcome the friction, the tool face angle may overshoot its target, thereby requiring the driller to apply a reverse angular correction.
It is known in the art that the frictional engagement between the drill string and the borehole can be reduced by rotating the drill string back and forth (“rocking”) between a first angle and a second angle measured at the earth's surface. By rocking the string, the stick/slip friction is reduced, thereby making it easier for the driller to control the weight on bit and make appropriate tool face angle corrections. A limitation to using surface angle alone as basis for rocking the drill string is that it does not account for the friction between the wall of the bore hole and the drill string. Rocking to a selected angle may either not reduce the friction sufficiently to be useful, or may exceed the friction torque of the drill string in the bore hole, thus unintentionally changing the tool face angle of the drilling motor. Further, rocking the tool face angle alone may result in motor stalling if too much weight is suddenly transferred to the bit as friction is overcome.